1. Field of the Invention
The present invention relates to a cylindrical roller bearing and a cage for cylindrical roller bearings. There are various types of cylindrical roller gearings, including the N type (inner ring with two ribs) NU type (outer ring with two ribs), NF type (inner ring with two ribs, outer ring with single rib), NJ type (inner ring with single rib, outer ring with two ribs), and NUP type (inner ring with two ribs, of which one rib is constituted by a separate rib ring, outer ring with two ribs).
2. Description of the Prior Art
For example, main shaft devices for machine tools, such as machining centers, CNC laths, and milling machine, are often operated in high speed rotation for reasons including one intended to increase work machining efficiency and accuracy. Particularly nowadays, the trend toward speeding up by main shaft rotation speed is remarkable.
Generally, in a main shaft device for machine tools, the main shaft is supported for rotation relative to a housing in rolling bearings disposed on the front side (tool side) and rear side (counter tool side), and the rolling bearings are lubricated by such lubrication systems as oil mist lubrication, air oil lubrication, jet lubrication, and grease lubrication, depending on usage conditions or the like. Normally, the rolling bearing on the front side has a construction that does not allow axial displacement of the main shaft (fixed side), while the rolling bearing on the rear side has a construction that allows axial displacement of the main shaft (free side) in order to absorb or relieve the amount of axial expansion of the main shaft due to the heat produced during operation. Such rolling bearing used on the front side is often in the form of a combination angular ball bearing or combination angular ball bearing+double row cylindrical roller bearing, while the rolling bearing used on the rear side is often in the form of a combination angular ball bearing or double row or single low cylindrical roller bearing.
FIG. 10 shows a structural example of a cylindrical roller bearing. This cylindrical roller bearing comprises an inner ring 1 having a raceway surface 1a in the outer periphery, an outer ring 2 having a raceway surface 2a in the inner periphery, a plurality of cylindrical rollers 3 rollably disposed between the raceway surface 1a of the inner ring 1 and the raceway surface 2a of the outer ring 2, and a cage 4 for holding the cylindrical rollers 3 at predetermined circumferentially spaced intervals. The opposite sides of the inner ring 1 are each provided with a rib 1b. 
There are two guide (positioning) types for the cage: an outer ring or inner ring guide type for guiding the cage by the inner peripheral surface of the outer ring or the outer peripheral surface of the inner ring, and a roller guide type for guiding the cage by rollers. In the case of the roller guided cage, during high speed rotation the cage, under the influence of centrifugal force, tends to whirl or tends to be deformed under excessive load from the rollers, resulting in a break (see Japanese Patent Unexamined Specification 2002-323048).
With the roller guide types for the cage, in order to reduce whirling, the radial clearance δ (diametrical clearance) between the cage pocket and the cylindrical rollers has been set small. For example, the ratio of the clearance to the diameter Dw of the cylindrical rollers, δ/Dw, is set to 0.01-0.10. Particularly, in the case of high speed rotation, it is set to 0.01-0.05.
In the meantime, the cylindrical roller bearing for the main shaft of a machine tool, for example, aims at reducing the radial internal clearance after assembly to zero in order to attain processing of high accuracy and to suppress the chattering of the main shaft. During operation, the inner ring temperature is higher than the outer ring temperature, in which case the amount of expansion of the inner ring is larger than that of the outer ring, so that the initial radial internal clearance further reduces to a negative value (preloaded state).
Generally, in the bearing in operation, the rolling elements do not advance at given intervals under the influence of the dimensions and shapes of the rolling elements, cage and inner and outer rings; rather, some lead and some lag. In the case where there is a radial internal clearance, even if such lag-lead occurs in the loaded region, it is relieved in the clearance region, so that there is no possibility of the force (difference between lag and lead) building up. In the negative clearance state, however, since there is no region for relief, the generated lag-lead builds up, and its force influences the cage.
The force acting on the cage from the rollers due to lag-lead can cause deformation of the entire annulus of the cage, not to mention the deformation of the tongue pieces of the cage contacting the rollers. If the spring force due to deformation of the cage (which spring force corresponds, for example, to the force by which the annulus of the cage returns to the original state after it is deformed to oval, or the force by which the tongue pieces return to the original state after they are deformed toward the column middle by the force from the rollers) becomes greater than the force from the rollers, a slip occurs between the rolling elements and the raceway surfaces, whereby the buildup of force is temporarily relieved. In the case where no consideration is given to the strength aspect of the cage, however, the cage cannot withstand repetition of such deformation and relief during operation, possibly resulting in a break.
Operating conditions for modern machine tools require speeding up, of course, and reaching maximum rpm in a short time and stopping in a short time (quick acceleration and quick stoppage), in order to shorten the jig exchange time, so that the force acting on the cage is on the increase.
Further, in order to cope with high accuracy and high speed operation, there are cases where the housing is cooled. Though having the advantage of the bearing temperature being lowered, this positively cools the outer ring to produce a greater difference in temperature between the inner and outer rings, causing the radial internal clearance during operation to become a negative clearance (increase in preload), which is also a main cause of increasing the force acting on the cage.